I. Field of the Invention
This invention relates generally to a method of implementing the hardware and processing the resultant output data of a delta-sigma analog to digital (A/D) converter. The manner by which this is accomplished improves the resolution, accuracy, dynamic range, noise level, and conversion speed while reducing the hardware requirements and cost significantly.
II. Discussion of the Prior Art
Delta-sigma A/D converters are a class of A/D converters characterized by their extremely simple quantizer design. The quantizer within a .DELTA.-.SIGMA. A/D converter provides an output signal having only one bit of resolution. To achieve the desired resolution, many sequential output samples from the quantizer are averaged or processed by other digital means. The primary advantage of .DELTA.-.SIGMA. A/D converters is their analog simplicity. It makes them exceptionally linear, small, and insensitive to component tolerance variations, all of which makes them easy to integrate within a mostly digital integrated circuit.
A simple .DELTA.-.SIGMA. A/D converter typically comprises a first operational amplifier having the analog input to be converted applied to its non-inverting input and a reference voltage applied through an electronic switch to its inverting input. A feedback capacitor couples the output of this operational amplifier back to its inverting input and, as such, the combination functions as an analog integrator. The output of the integrator circuit is then effectively coupled to the data input of a D-type flip-flop which is clocked at a predetermined rate. When the output from the integrator exceeds the threshold established for the D-input of the flip-flop, a logical "1" output will appear at its output terminal at the time that a clock signal is applied to the clock input of that D-type flip-flop. The output from the flip-flop comprises a serial digital data output stream and that output data stream is also effectively applied to the inverting input of the op-amp integrator.
To date, .DELTA.-.SIGMA. A/D converters have found applications mainly in the telecommunications and digital audio field. The most prevalent application of this is in compact disc "CD" players. Other applications of this technology have been in video digitization for combined audio-video telephone service. More limited use of the .DELTA.-.SIGMA. A/D conversion has been made in applications requiring accurate measurement of DC and low frequency AC signals riding a DC component.
Prior art .DELTA.-.SIGMA. A/D converters are characterized by a source of analog information which is fed into a .DELTA.-.SIGMA. A/D quantizer which converts the analog data into a "density" modulated serial digital data stream, and subsequent digital processing of that data to arrive at a meaningful output. Within the quantizer, there are one or more stages of analog integration, which corresponds to the "order" of the quantizer. This is followed by a single stage digital delay element, typically, a latch, which provides the output from the quantizer as well as a portion of the feedback signal for the quantizer.
The output of the quantizer in prior art .DELTA.-.SIGMA. A/D converters has been processed in a number of ways including decimation, averaging, and digital filtering to arrive at a usable signal. These methods have been used in various combinations and have been combined with weighting of the decimated samples and dithering to improve the attainable resolution (and hence the ultimately realizable accuracy and signal-to-noise ratio) and conversion speed.
Although there are many advantages to implementing the .DELTA.-.SIGMA. A/D conversion process over more conventional A/D conversion processes, such as flash and successive approximation, the prior art methods of .DELTA.-.SIGMA. A/D conversion unfortunately suffer from a much longer data latency than can be tolerated in many applications, most notably in control systems.
A goal of prior art .DELTA.-.SIGMA. A/D converters has always been to arrive at a means of configuring a .DELTA.-.SIGMA. quantizer and processing its serial output to achieve the highest resolution, and hence best dynamic range, in the shortest possible time, thereby maximizing the frequency response. The present invention extends that state of the art.
This invention is based on .DELTA.-.SIGMA. A/D conversion principles, however, the implementation of the hardware and data reduction process have been modified to provide a significant advance in terms of cost reduction through the ability to use components having lower gain/bandwidth characteristics and in terms of increased accuracy. It is those hardware modifications and the new output data reduction implementation which are the focus of this invention.